The present invention relates to sheet coil motors and the method of fabricating the same, and more particularly to a sheet coil motor constructed to be thin and the method of fabricating the same at a reduced cost.
In a recording/reproducing apparatus such as a magnetic disk drive and the like, a disk cartridge accommodating a flexible magnetic disk (recording medium) is inserted into a disk insertion opening provided on a front bezel of the apparatus. As the disk cartridge is inserted into the apparatus, the flexible magnetic disk is mounted on a turntable so as to be ready for recording/reproducing operation.
Efforts are made to reduce the thickness of such an apparatus. In one approach, a sheet coil motor is employed for rotating the flexible magnetic disk mounted on the turntable.
FIG. 1 is a longitudinal cross sectional view showing the construction of a conventional sheet coil motor 1A. The sheet coil motor 1A shown in FIG.1 is a direct drive motor of an axial gap type (parallel type).
The sheet coil motor 1A is provided on a circuit substrate 2. The sheet coil motor 1A has a driving sheet coil 3 formed by a plurality of coils in an annular arrangement, a rotor magnet 4 provided opposite the driving sheet coil 3 at a close distance, and Hall elements 5 provided adjacent the driving sheet coil 3 so as to detect the variation in magnetic field. The driving sheet coil 3 is provided opposite the rotor magnet 4 at a close distance.
The rotor magnet 4 is fixed on the underside of a turntable 6 on which a flexible magnetic disk (not shown) is mounted. The turntable 6 is rotatably supported, via a ball bearing 7, by a spindle 8 erected on the circuit substrate 2.
Three Hall elements 5 are soldered to the circuit substrate 2 to constitute a driving circuit (not shown) for effecting phase switching. The circuit substrate 2 is made of an iron and is provided with the driving sheet coil 3 on the upper surface thereof.
The sheet coil motor 1A having the above described construction is configured such that an electromotive force is generated between the driving sheet coil 3 and the rotor magnet 4 by allowing a current to pass through the driving sheet coil 3, and energized phases are switched between each other as the Hall elements 5 detect the electromotive force (magnetic field), thereby maintaining the rotor magnet 4 in rotation. In other words, the driving circuit switches in and out phases produced by respective coils provided in the driving sheet coil 3, on the basis of a detection signal supplied from the Hall elements 5 so that the flexible magnetic disk is rotated at a predetermined constant speed required for recording/reproduction.
As shown in FIG.2, in the sheet coil motor 1A, coils 3.sub.1 -3.sub.12 of the driving sheet coil 3 are provided at a regular interval in an annular arrangement so as to cover 360.degree. of a circle. Alternatively, as shown in FIG.3, coils 3.sub.1 -3.sub.9 may be provided at a regular interval in an annular arrangement so as to cover 270.degree. of a circle.
The plurality of coils 3.sub.1 -3.sub.12 or the coils 3.sub.1 -3.sub.9 of the conventional driving sheet coil 3 are printed on the substrate as a set. For example, as shown in FIG.4, six driving sheet coils 3 each including an annular arrangement of the coils 3.sub.1 -3.sub.12 are printed on a sheet base 9, and each driving sheet coil 3 is cut out from the sheet base 9. Accordingly, only a relatively small number of driving sheet coils 3 can be provided in a given area on the sheet base 9. Hence, there is a problem in that the number of coils that can be printed on and cut out from a sheet base is relatively small, and in that a relatively large portion not used for the driving sheet coils 3 is wasted, thereby reducing the efficiency of the production.
In a conceivable construction to resolve the above problems, sheet Coils having a shape of a sector are concentrically arranged and soldered on the circuit substrate.
As shown in FIG.5, in a conceivable sheet coil motor 1B of the above described construction, connecting terminals 10 of the coils are disposed in the intervals between the coils. The connecting terminals 10 of the coils are connected by soldering to connecting terminals (not shown) formed in the circuit substrate 2.
In the conceivable sheet coil motor 1B, solder 10d which is put on the connecting terminal 10 projects above the driving sheet coil 3. Therefore, it is necessary to provide a large distance L between the rotor magnet 4 and the driving sheet coil 3 so that the rotor magnet 4 does not come into contact with the solder 10d.
Hence, an effort to reduce the thickness of the motor is thwarted by the solder 10d provided as a result of soldering the connecting terminal 10 of the driving sheet coil 3 to the connecting terminal (not shown) of the circuit substrate 2. Another problem is that inspection of the connection between the connecting terminal 10 and the circuit substrate 2 is performed while the turntable 6 is detached from the circuit substrate 2. Such an inspection is troublesome and prevents efficient maintenance from being performed.
FIG.6 shows another conceivable sheet coil motor 1C, where the driving sheet coil 3 can be as thin as 0.3 mm. However, the height of 0.7 mm of the Hall elements 5 is necessary in order to ensure that the Hall elements detect the magnetic field with a sufficient precision for rotation control.
A relatively large separation L is to be provided between the driving sheet coil 3 and the rotor magnet 4 in order to prevent the rotor magnet 4 and the Hall elements 5 from coming into contact with each other. As a result, not only the reduction of the thickness of the motor can not be achieved, but the driving force of the motor is small, and it is impossible to obtain a satisfactorily large driving torque.
Hence, there is a problem in that an expensive sintered magnet is needed to form the rotor magnet 4 in order to ensure that the magnetic flux density sufficient for the driving of the rotor magnet 4 at a predetermined constant speed is obtained.